This work aims to study the behavior of the western margin of the Greenland Ice Sheet during a period of pronounced ice retreat roughly 10,000 years ago, after the end of the last glacial period. It explores the efficiency of subglacial erosion, the spatial dynamics of ice retreat, and the rates of ice retreat. To address these questions, I use the radionuclides 10Be and 26Al, which form in rocks due to the bombardment of cosmic rays, only after the rocks have been exposed from underneath retreating ice. These nuclides can be used as a geologic dating technique to explore exposure history. Before applying this dating technique to address geological questions, it was critical to first perform methodological development. My work in the University of Vermont‘s new Cosmogenic Nuclide Laboratory served to improve the precision and efficiency of the pre-existing laboratory methods. New methodological advances ensured that samples from Greenland, which contained only low concentrations of 10Be and 26Al, could be used to yield meaningful results about ice behavior. Cosmogenic nuclide dating was applied at two sites along the ice sheet margin in central-western Greenland. At both of these sites, I collected paired bedrock and boulder samples in a transect normal to and outside of the present-day ice sheet margin. Samples were collected from a variety of elevations at numerous locations along the transects, thus providing three-dimensional coverage of the field area. After isolating the mineral quartz from the rocks, and isolating the elements Be and Al from the quartz, isotopic analysis was performed using accelerator mass spectrometry to quantify the relative abundances of the radionuclides against their respective stable isotopes. The southern study site, Ilulissat, is located on the western coast of Greenland at a latitude of 69N. Much previous work has been conducted here due to the presence of one of the largest ice streams in the northern hemisphere, Jakobshavn Isbræ. My work in Ilulissat demonstrated that subglacial erosion rates were high during previous glacial periods, efficiently sculpting and eroding the landscape. Ice retreat across the land surface began around 10,300 years ago, and the ice sheet retreated behind its present-day margin about 7,600 years ago. Ice retreat occurred at a rate of about 100 meters per year. My work in this area suggests that retreat in the large ice stream set the pace and timing for retreat of the neighboring ice sheet margin. The northern site, Upernavik, is located on the western coast of Greenland at a latitude of 73N. Little research has been conducted here in the past. Unlike in Ilulissat, my work here shows that the ice sheet did not efficiently erode the landscape, especially at high elevations, during previous glacial periods. This is likely because the ice was thinner, and therefore had a colder base, than the ice in Ilulissat. My work suggests that ice cover was lost from this area very rapidly, likely at rates of about 170 meters per year, in a single episode around 11,300 years ago. Comparison between the two study sites reveals that ice characteristics can vary appreciably over relatively small distances.

Thesis advisor: Jeremy D. Shakun / The deglacial extent chronology of the southeastern Laurentide Ice Sheet as it retreated through the northeastern United States and southern Quebec has been well constrained by multiple lines of evidence. By comparison, few data exist to constrain the thinning history of the southeastern Laurentide, resulting in lingering uncertainty about volume changes and dynamics of this ice mass during the deglacial period. To address the lack of thinning information, my team collected 120 samples for in-situ `10Be exposure dating from various elevations at numerous mountains in New England and southern Quebec. Monte Carlo regression analyses using the analytical uncertainties of exposure ages from each mountain are used to determine the most-likely timing and rate of ice thinning for that location, a technique known as the ‘dipstick approach’. While this larger project is ongoing, I have processed and measured 10Be concentrations of 42 samples for this thesis and present my preliminary results and interpretation here. Exposure ages from Peekamoose Mt. in southern NY suggest ice thinning early in the deglacial period (~19.5 – 17.5 ka), near the onset of the Heinrich Stadial I cold period. Samples from Franconia Notch, NH, and Mt. Mansfield, VT, suggest ice thinning from approximately 15 – 13 ka in northern New England, roughly coincident with the Bølling-Allerød warm period. Exposure ages from each of the northern New England dipsticks are nearly identical within 1σ internal uncertainty, indicating that ice thinning was rapid. Higher elevation (>1200 m a.s.l.) samples from the northern New England mountains appear to contain inherited 10Be from previous periods of exposure, indicating a lack of glacial erosion on these surfaces. My high-elevation samples with inherited 10Be suggest that these summit landscapes were preserved beneath cold-based, non-erosive ice during the last glacial and deglacial periods. 40 samples that have yet to be processed will provide more information on ice thinning around Killington Mt., VT, Mt. Greylock, MA, Mt. Bigelow, ME, and Mt. Jacques-Cartier, Quebec. Ultimately, this information will be used to create probabilistic reconstructions of the lowering southeastern Laurentide ice surface during its retreat. / Thesis (MS) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

The plate-boundary Alpine Fault runs immediately offshore of the popular tourist destination of Milford Sound, which is visited by more than half a million tourists each year. Glaciers retreated from the fiord between ~24-16 ka, leaving behind a legacy of extreme topography, including some of the world's highest sea cliffs, which tower nearly 2 km above the fiord. Visitors come to view the spectacularly steep and rugged landscape, with many cruising the fiord by boat. This project utilizes surface exposure dating (TCND) of glacially modified surfaces, to gain further insight into the glacier retreat history of Milford Sound. Exposure dates from strategic locations near the entrance to the fiord indicate that the main trunk glacier had retreated about 9 km from its peak LGM position by ~18 ka. Additional TCND and calibrated Schmidt Hammer data from a range of positions within the Milford catchment provide strong evidence that the main trunk glacier receded rapidly after about 18 ka, retreating a further 16 km to a position near the present-day confluence of the Tutoko and Cleddau rivers, by ~16 ka. Available seismic reflection data suggest that post-glacial sediment infill has been strongly influenced by massive deposits of rock avalanche debris. New high-resolution bathymetric and seismic reflection data reveals the presence of at least 18 very large post-glacial rock avalanche deposits which blanket ~40% of the fiord bottom. Geomorphic mapping and field investigation reveal the presence of at least ten additional very large to giant terrestrial landslide deposits in the lower Milford catchment; radiocarbon and surface exposure dating indicate that these events occurred during the Holocene, between ~9-1 ka. Ages of six of these deposits are in agreement with published rupture dates on the southern on-shore portion of the Alpine Fault.